1. Field of the Invention
This invention relates to a liquid crystal device using polymer dispersed liquid crystal.
2. Related Background Art
Since conventional liquid crystal panels (liquid crystal devices) require a polarizing plate for uniformizing polarization direction of light, more than a half of the quantity of incident light is absorbed as incident light passes through the polarizing plate to significantly reduce the luminance of the obtained image.
Contrary to this, light scattering type liquid crystal panels comprising polymer dispersed liquid crystal prepared by combining liquid crystal and a polymer material do not require any polarizing plate and hence is free from the problem of reduced luminance of the obtained image. For this reason, massive efforts are currently being paid in the development of liquid crystal panels of this type.
Meanwhile, polymer dispersed liquid crystal is classified as either of two major types depending on the dispersed state of liquid crystal and that of the polymer material. One is PDLC (polymer dispersed liquid crystal) as used in the proper sense of the word, where minute drops of liquid crystal are dispersed in the polymer material and hence liquid crystal is found in a discontinuous state and the other is PNLC (polymer network liquid crystal) where the polymer material forms a network and liquid crystal is found In a continuous state.
While the two types do not differ from each other in the sense that light is transmitted and scattered by liquid crystal depending on the voltage applied thereto, scatter of light occurs on respective principles that are different from each other. More specifically, PDLC is used to exploit its property that the refractive index varies depending on the direction of orientation of liquid crystal. When no voltage is applied, the liquid crystal of each drop is oriented in its own direction that is not particularly defined. Then, the refractive index of liquid crystal and that of the polymer material differ from each other and hence incident light is scattered. However, when a voltage is applied, all the liquid crystal becomes to be oriented in a same direction. If the polymer material is so selected that its refractive index agrees with that of the liquid crystal when the latter is oriented in a same direction, incident light is transmitted through PDLC.
On the other hand, when no electric field is applied to PNLC, liquid crystal is arranged along the walls of the network of the polymer material so that it is oriented randomly and hence light is scattered by it. However, when a voltage is applied, the molecules of liquid crystal become oriented uniformly so that light is transmitted through PNLC. Therefore, while scatter of light in the inside is largely dependent on the difference between the refractive index of the liquid crystal and that of the polymer for both PNLC and PDLC, the network structure of the polymer also greatly affects the performance. The threshold voltage of PNLC can be made lower than that of PDLC (the ratio of the voltage applied to the liquid crystal of PNLC is higher than that of the voltage applied to the liquid crystal of PDLC when the overall voltage is same for both PNLC and PDLC) and PNLC responses more quickly to the applied voltage than PDLC. This is the reason why PNLC is attracting more attention more than PDLC.
Both PDLC and PNLC can be prepared with ease in a short period of time by uniformly mixing and dissolving ultraviolet-rays-setting type resin and liquid crystal and irradiating ultraviolet rays to the mixture. PDLC is produced when the content ratio of the ultraviolet-rays-setting type resin is raised, whereas PNLC is formed by reducing the content ratio. The size of the voids of the network structure of polymer can be controlled by selecting an appropriate ultraviolet-rays-setting type resin and/or by controlling the intensity of irradiated ultraviolet rays.
Japanese Patent Application Laid-Open No. 2-203319 discloses a method of preparing PDLC by using ultraviolet-rays-setting resin and Japanese Patent Application Laid-Open No. 2-207220 describes a method of preparing PNLC by using ultraviolet-rays-setting resin.
Japanese Patent Application Laid-Open No. 8-015675 discloses PNLC comprising ultraviolet-rays-setting type resin, the PNLC revealing a memory property which can be erased by applying heat thereto.
A polymer dispersed liquid crystal containing cholesteric liquid crystal and showing a memory property both in the focalconic orientation and in the planar orientation is disclosed in Japanese Patent Application Laid-Open No. 11-149088 and other patent documents. However, since such a device involves twisted orientation of liquid crystal molecules, the absolute amount of light transmitted through cholesteric liquid crystal is small if compared with nematic liquid crystal due to a large loss and hence a high contrast effect cannot be expected from it.
Unlike the above listed liquid crystal panels, Japanese Patent Application Laid-Open No. 8-062586 discloses a liquid crystal panel designed to utilize double refraction of liquid crystal. The liquid crystal panel is provided with a polarizating plate and two pairs of electrodes for forming respective electric fields in different directions so that transmission/non-transmission of light is controlled by appropriately shifting the directions of the electric fields.
Japanese Patent Application Laid-Open No. 2000-284264 discloses a method for improving the light scattering efficiency of a liquid crystal panel by orienting polymer showing refractive index anisotropy. With this method, the light scattering efficiency of a liquid crystal panel can be improved by using polymer precursors having a liquid crystal property and forming a polymer arrangement so as to make it show optical anisotropy.
However, conventional PDLC and PNLC adapted to utilize scatter of light are accompanied by the problem of an insufficient intensity of scattering because the orientation of liquid crystal is random when scattering light.
With the technique of erasing the memory state by applying heat (Japanese Patent Application Laid-Open No. 8-015675), it is not possible to accurately control the area to be erased and the timing of erasing and ensure a high image quality for the displayed image.
None of the known liquid crystal devices do not show a memory property both in the transparent state and in the opaque state and they mostly consume power at a high rate.
Additionally, a liquid crystal panel disclosed in Japanese Patent Application Laid-Open No. 8-062586 is accompanied by the problem of a low efficiency of light utilization mainly because it uses a polarizating plate. Still additionally, it requires the use of a back light to make it a poorly power saving device.
The method disclosed in Japanese Patent Application Laid-Open No. 2000-284264 requires a special orienting technique of using in combination a special material as polymer precursor that shows a nematic phase by itself and can be polymerized without disturbing the orientation of liquid crystal and a uniaxial orienting treatment such as rubbing.
In view of the above identified circumstances, it is therefore an object of the present invention to improve the photo-conversion efficiency of conventional PDLC and PNLC, the scattering efficiency in particular.
Another object of the present invention is to provide a liquid crystal device having a simple configuration and showing an improved scattering efficiency without resorting to a special material and/or a special technique as disclosed in Japanese Patent Application Laid-Open No. 2000-284264.
Still another object of the present invention is to provide a liquid crystal device that can prevent a bad image quality from occurring.
Still another object of the present invention Is to provide a liquid crystal device that serves to save power.
A further object of the present invention is to provide a liquid crystal device that provides a high contrast effect.
According to the invention, the above objects and other objects are achieved by providing a liquid crystal device comprising:
a pair of substrates arranged vis-a-vis with a predetermined gap separating them;
polymer dispersed liquid crystal formed by dispersing liquid crystal in a polymer material and arranged in the gap separating the pair of substrates;
first electrodes for forming an electric field in a first direction relative to said polymer dispersed liquid crystal; and
second electrodes for forming an electric field in a second direction relative to said polymer dispersed liquid crystal, said second direction being perpendicular to said first direction.
Preferably, in a liquid crystal device according to the invention and comprising such a pair of substrates, the liquid crystal is oriented independently in two directions rectangularly intersecting each other.